- What is your measured value?
- Choose which number you have. If the amplifier in strain-gauge mode already shows a strain in µm/m (micrometres per metre; 1000 µm/m = 0.1 % change of length), choose strain. If it only shows the raw Wheatstone-bridge signal in mV/V (millivolts of output per volt of excitation), choose signal; gauge factor and bridge circuit are then needed for the conversion.
- Measured longitudinal strain ε_l [µm/m]
- The strain shown by a gauge bonded along the rod. Positive means the rod gets longer (tension), negative shorter (compression). For a full bridge this is the strain of one single longitudinal gauge, not the sum of all four. Source: the amplifier reading after zero balancing with the rod unloaded.
- Bridge signal U_M/U_B [mV/V]
- The raw bridge signal: Wheatstone-bridge output divided by the excitation, in mV/V. Keil's support rod gives about 1.29 mV/V at 10 kN. Source: the amplifier reading in mV/V mode after zero balancing.
- Gauge factor k
- The gauge factor is the sensitivity of the strain gauge: it states how much the electrical resistance changes when the gauge is stretched (ΔR/R = k·ε). Without it no strain can be computed from an electrical signal. Source: printed on every gauge package or in the manufacturer datasheet, typically 2.0 to 2.1 for constantan foil gauges; valid at room temperature. The same value must be set in the amplifier.
- Bridge circuit
- How the gauges are wired – this sets the bridge factor B, i.e. how many times the longitudinal strain is contained in the signal. One gauge: B = 1. One longitudinal and one transverse gauge: the transverse gauge measures the contraction −ν·ε and raises the signal to B = 1+ν. Two opposite longitudinal gauges plus two compensation gauges: B = 2, bending cancels. Two longitudinal and two transverse (Keil's measuring element): B = 2(1+ν) ≈ 2.58, bending- and temperature-compensated. Source: your own wiring or the element's datasheet.
- Cross-section
- How the load-bearing area at the gauge location is computed: circle from the diameter, rectangle from width times thickness, or you enter the area directly (e.g. from a drawing, for a tube or a profile). Only the section exactly where the gauges are bonded counts.
- Diameter d [mm]
- The rod diameter at the gauge location, circle only. The area grows with the square – a 1 % error in diameter is a 2 % error in force. Source: calliper; for threaded rods the shank diameter at the gauge location, not the thread core.
- Width b [mm]
- The width of the rectangular section at the gauge location (rectangle only). For flat bars the dimension the gauge is bonded on. Source: calliper or drawing.
- Thickness h [mm]
- The thickness of the rectangular section at the gauge location (rectangle only), i.e. the dimension perpendicular to the bonding surface. Source: calliper or drawing.
- Cross-section area A [mm²]
- The cross-section area directly in mm², only when ‘enter area directly’ is selected. Useful for tubes (π/4·(d_o²−d_i²)), profiles or values from the drawing. Keil's example: 8 mm round → 50.3 mm².
- Young's modulus E [N/mm²]
- Young's modulus describes how stiff the part's material is – how much stress it takes to produce a given strain (σ = E·ε). It is needed to turn measured or computed strains into stresses and vice versa. Source: material tables or datasheet; steel ≈ 210 000 N/mm², aluminium ≈ 70 000 N/mm², titanium ≈ 110 000 N/mm². E drops at elevated temperature.
- Poisson's ratio ν
- Poisson's ratio states how much a material contracts transversely when stretched longitudinally: ε_transverse = −ν·ε_longitudinal. It matters because transversely bonded gauges measure exactly this contraction and because both directions interact in biaxial stress states. Source: material tables; steel 0.28–0.30, aluminium 0.33, plastics 0.35–0.45.